They don't mention it on their website anymore, but historically, they've held it on Shrove Tuesday.

Known also as Fat Tuesday or Mardi Gras, National Pancake Day dates back
several centuries to when the English prepped for fasting during Lent.
Strict rules prohibited the eating of all dairy products during Lent, so
pancakes were made to use up the supply of eggs, milk, butter and other
dairy products...hence the name Pancake Tuesday or Shrove Tuesday